A trend in personal communication devices (e.g., cellular telephones and personal digital assistants) is to make the devices more capable by adding more hardware. Cameras are common on these devices, as are multiple display screens. GPS receivers are becoming more common. Other devices for monitoring either the environment around the device (e.g., a thermometer) or for monitoring the user of the device (e.g., a heartbeat detector) have been proposed.
Useful as this additional hardware can be in particular situations, much of it is used only rarely. The cost of the hardware is paid for (directly or indirectly) by the user, but the hardware provides only a very limited return on that cost. Ready to perform, this hardware spends most of the time taking up space in the device, adding weight, cost, and operational complexity, and only occasionally providing a service to the device's user.